The present invention relates to an image forming apparatus, and specifically relates to a color image forming apparatus having an intermediate transfer member onto which a toner image is transferred.
As a color image forming apparatus, which employs an intermediate transfer member, there has been well known such an apparatus in which a toner image formed on an image bearing member is transferred onto an intermediate transfer member, and then, the toner image transferred onto the intermediate transfer member is further transferred onto a transfer material. In the color image forming apparatus of this type, the toner image, formed on the image bearing member and charged at a predetermined polarity, is transferred onto the intermediate transfer member by using an electrostatic force, and then, the toner image residing on the intermediate transfer member is further transferred onto a transfer material by using an electrostatic force.
Specifically, since it is possible for the abovementioned color image forming apparatus to make plural toner images, formed on the image bearing member, overlap with each other on the intermediate transfer member, the apparatus is widely employed as a color image forming apparatus for forming a color image on the transfer material. In the color image forming apparatus of this type, unicolor toner images, each of which is formed on an image bearing member corresponding to each of the primary colors, are sequentially transferred one by one while overlapping with each other on the intermediate transfer member so as to form a full color toner image thereon, and then, the full color toner image is further transferred onto the transfer material at a time by means of a electrostatic force.
Since an amount of electronic charge per one toner particle is substantially uniform, an electronic potential in the toner image residing on the intermediate transfer member is determined by an amount of toner adhered within a predetermined area. Accordingly, in the color image forming apparatus, within the full color toner image formed on the intermediate transfer member, a charged potential of an area at which plural color toners overlap with each other is greater than that of another area at which only a single color toner resides. Further, for instance, when the abovementioned full color toner image formed on the intermediate transfer member includes both a high density area and a halftone area, a charged potential of the high density area is greater than that of the halftone area.
Further, sometimes, variations of the charged potential within the toner image, passed through the primary transferring section for transferring the toner image from the image bearing member to the intermediate transfer member, would occur depending on environmental factors.
The fact that the variations of the charged potential widely vary within the toner image, as mentioned in the above, means that plural areas being different from each other in its transferring characteristics coexist within the same toner image. If it is tried to transfer all of such the plural areas, being different from each other in its transferring characteristics, onto the transfer material under the same transferring conditions, various kinds of image deficiencies are liable to occur at the time of the secondary transferring operation from the intermediate transfer member to the transfer material.
In recent years, colorization trends have been proliferated in the filed of imaging apparatus including a copier, a facsimile, a compound image forming apparatus having such the functions of them, and further, associated with employments of the polymerized toner and small-sized toner particles, demands for high quality imaging have largely increased in the field of transferring process. In addition, speedup trends of the abovementioned image forming apparatus have been progressing as well. In order to produce a good image according to the abovementioned trends, it is necessary to compensate for the electronic potential of the toner image residing on the intermediate transfer member, which varies with a number of the primary transferring operations and the environmental factors, at a substantially a uniform potential, so as to improve the secondary transferring efficiency.
To overcome the abovementioned problem in conventional image forming apparatus, there has been proposed such a configuration that a conductive roller member, serving as a pre-charging device for charging a toner image primary-transferred on the intermediate transfer member before transferring it onto a transfer material, is disposed at a back side of the intermediate transfer member, which is opposed to a charging device and electrodes of the charging device, so as to work as an opposed electrode (for instance, set forth in Patent Document 1). According to this configuration, the charging operation conducted by means of a corona discharging action of AC (Alternate Current), DC (Direct Current), etc., is applied to the toner image transferred onto the intermediate transfer member, so as to make the charge amount over the toner image substantially uniform.
Further, it has been also proposed to provide a controlling apparatus for controlling a charging condition for the pre-charging device, in response to the surface moving velocity of the intermediate transfer member passing through the charging position of the pre-charging device (for instance, set forth in Patent Document 2).
[Patent Document 1]                Tokkaihei 10-274892 (Japanese Non-Examined Patent Publication)        
[Patent Document 2]                Tokkaihei 11-143255 (Japanese Non-Examined Patent Publication)        
According to the methods described in Patent Documents 1 and 2, since the charge amount of the toner image on the intermediate transfer member is set at a relatively large value as the uniform potential, when the resistivity of the paper, serving as the transfer material, is high due to a low humidity environment or a transferring operation of the second surface of the paper in the duplex copy mode, image deficiencies, due to discharging actions caused by the electronic potential rise of the paper, are liable to occur. If the transferring voltage were set at a low value in order to prevent such the image deficiency, an area, where the total charging amount is relatively large, would be suffered by a lack of transferring electronic field, resulting in an occurrence of the density unevenness.
Further, with respect to the configuration that the conductive roller member, serving as the pre-charging device, is disposed at the back side of the intermediate transfer member, which is opposed to the charging device, so as to work as the opposed electrode, it becomes difficult to acquire a sufficient charging effect according as the line velocity of the intermediate transfer member is getting large in accordance with the speedup trend of the image forming apparatus. In addition, since it is necessary to control the charging condition in response to the line velocity of the intermediate transfer member, the total configuration is getting complicated.